1. Field of the Invention
This invention relates to a process for producing a laminated capacitor made by stacks of metallized plastic films as well as the capacitor obtained by such a process.
2. Description of the Related Art
One of the steps of a known process consists in winding on a large diameter wheel at least one pair of metallized plastic films. Each metallized plastic film is in the form of a ribbon resulting from the cutting of a width of metallized plastic film of great width. During this winding phase, a nonmetallized lateral portion is formed on each of the two films of the pair. The nonmetallized lateral portions are made on opposite sides to obtain alternating layers of films consisting of an even row film and an odd row film.
Thus, a capacitive ribbon is obtained that has alternate even and odd row layers, called a mother capacitor. Each of the sides of the capacitive ribbon is then covered with a metal or an alloy to form terminals. Each terminal (output plate) makes it possible to mechanically and electrically connect the metallizations of the layers of the same row to one another. This operation, performed by spraying molten metal, is known as shooping. To improve the mechanical anchoring of the molten drops on the films of the same row, it is recommended to offset the even row films in relation to the odd row films.
The capacitive ribbon is then cut into elementary units. Each such unit constitutes a laminated capacitor of generally parallelepiped shape comprising six faces: two faces constituting the plates, two cutting faces and two faces perpendicular to both the plates and the cutting faces. To protect the two faces perpendicular to the plates and to the cutting faces, several solutions are known to a person skilled in the art. One of them consists of performing finishing turns at the beginning and at the end of the winding phase. The films are demetallized locally so that their winding does not present any capacitive effect on the final component.
The pulsed laser that is used to make the nonmetallized edges, whose width and positioning make it possible to define the capacitance of the capacitor, is generally also used for the local demetallization performed during the making of the finishing turns.
Conventionally, this laser acts by the spot thermal effect. It is necessary to modify the adjustment of the laser depending on whether it is desired to make nonmetallized side portions or perform said local demetallization.
The local demetallization is performed at the beginning and at the end of the winding phase, i.e., with a speed of winding of the metallized plastic films that is less than the established operating speed. Thus, unless either the frequency or the width of the pulses emitted by the laser, or the energy transmitted, is reduced, there is deterioration and burning of the plastic films.
This modification of the adjustment parameters of the laser results in interrupting its operation two times, a first time at the beginning of winding and a second time at the end of winding.
The production process is burdened by these various manipulations.
On the other hand, the laser beam, acting in a significant way by thermal effect, causes in every case a heating of the plastic films. This heating results in a local deterioration of dielectric rigidity as well as the voltage capacity of the capacitor. It can also be a source of deformations of the films and, therefore, lead to coiling problems.
Another drawback of the demetallization process using the thermal effect according to the prior art is the persistent presence of residual metallizations resulting mainly from remelting in the zones supposed to have been demetallized.